Artificial lighting is a prerequisite for human societies of today, and the properties and quality of the lighting has a great impact on the well-being of the people who are exposed. A vast terminology and set of parameters are used to define light and lighting, but the following non-exhaustive list can be used to describe the light source itself.
Natural sunlight has a continuous white light spectrum. The color rendering index, CRI, of a light source is a quantitative measure of its ability to replicate the colors of various objects faithfully in comparison with natural sunlight. It is measured as a percentage. This means that a CRI of 100%, taken at a color temperature of noon sunlight, approximately 6000 K, will exactly reproduce the colors found on a sunny day at noon. Light sources with a high CRI are desirable in color-critical applications such as e.g. photography or inspection of processing in e.g. the food industry.
Optical phenomena are also influencing perception of light and lighting. The document DE 10237751 A1 refers to the “slowness of the eye” as the phenomenon responsible for the eye not perceiving the flickering of the light source. This is a common but unfortunate misunderstanding. That is the “apparent movement” phenomenon in action, which makes still pictures, projected one after another at a certain frequency appear to move, as in “the movies”. This phenomenon occurs at about 25-30 Hz. However, anyone who has ever seen a silent movie knows that the light intensity is still visibly flickering at that frequency. The slowness of the eye is what makes the eye unable to perceive the variable intensity of a light source flickering with a certain frequency. The critical flickering frequency, CFF, occurs at about 60 Hz. However, even though the eye may not be able to perceive the flickering, “visual evoked potentials”, can occur, in which case flickering over the CFF may still be very tiresome for the brain and the nervous system. The usual fluorescent lamp run on 50-60 Hz can be very tiresome and is unsuited for workplaces e.g. According to the certification TCO'99©, the refresh frequency of a computer screen must be ≧85 Hz.
Somewhere between 80-120 Hz the visual evoked potentials disappear. All limits are individual and dependant upon e.g. age and health status, and also what kind of task is performed. A lighting that is truly healthy for all people and appropriate for all kinds of tasks should therefore exceed 120 Hz.
Besides these physical and physiological properties and limitations, certain EU directives, such as the Restriction of Hazardous Substances (RoHS) and the Waste Electrical and Electronic Equipment (WEEE) directives, put restrictions on electrical and electronical equipment, including equipment for lighting. For example, the RoHS directive forbids the use of e.g. lead and mercury. These directives are a part of the common EU law and constitute imperative demands on environmental compliance product development for the electronics industry.
Light pollution occurs when excessive amounts of artificial light is used, or when the light source is inadequately shielded, thus providing undesired lighting. Across the planet, the availability of so much artificial light has altered the habits of many animals. Many species are e.g. tricked into migrating early or late because of artificial lights interpreted to be sunlight, or starlight. The stray light also interferes with astronomers' possibility to make observations in populated areas. This unnecessary spreading of light can cost as much as 10 MUSD in the US alone, according to the International Dark-Sky Association.
A report released by the U.S. Department of Energy recommends that LED exterior lighting fixtures emit no light above 90°. The government of the Veneto region of northeastern Italy has prohibited upward-pointing lights.
The common, incandescent, light bulb produces immediate light, and because of its incandescence, it has a continuous spectrum and a CRI of 100%. From an economic and environmental point of view, the low light yield ˜5% and short lifetime are disadvantages. In addition, it must be waste sorted according to the WEEE. The halogen light bulb has a marginally higher light yield, but its light flux must be shielded and the bulb becomes very hot, and can thus not be placed anywhere because of the risk of fire.
The excitations of the fluorescent lamp generate UV light that must be converted to visual light through the Stokes shift. Further, the discontinuous spectrum must be compensated for, and the light usually must be shielded in order to be of use. Therefore the effective conversion of a fluorescent lamp is much lower than the theoretical 40%. In addition to this, fluorescent lamps contain mercury, which is listed in the RoHS directive. The so called compact fluorescent lamps feature a distinct delay of several minutes between power-on and full operation, and its conversion efficiency is down to some 25%, which makes it inappropriate for spaces such as restrooms or stairwells.
The light emitting diode or LED as it is henceforth called, has a conversion efficiency of some 50% and a lamp life of over 100 000 h. They do not contain hazardous substances listed in the RoHS or WEEE directives. A so called power LED can endure higher voltages and emit light with higher intensity than previous LED generations. LED's give immediate light; they are compact, robust and relatively inexpensive. Using the LED as a light source for lighting fixtures however, is impaired by a number of problems. First of all, an LED, contrary to a light bulb, does not give omnidirectional light, but the majority of its luminous flux radiates in a certain direction. Although a LED may lend itself to light indicators or selective lighting, generating an even flux of light over a large area is difficult with an LED.
The patent document WO 2008/108623A1 discloses a LED based lighting fixture with a standard socket, where the LED's, distributed over five facets of a stationary cube, are powered according to a certain sequence, aiming at emulating continuous non-directive lighting. It cannot emulate omnidirectional lighting, though, since there are no LED's in the sixth facet. Furthermore, there will always be a variation in the intensity distribution in different space angles, since the LED's are not sufficiently evenly or sufficiently densely distributed over the spherical coordinate system.
DE 10237751 discloses a lighting fixture using a white solid state laser as a light source. The light is guided through an optical system comprising mirrors and lenses. Mounted on the shaft of an electric motor, the optical system is rotated to emulate distributed lighting. White solid state lasers typically have a 10% conversion efficiency, which means that this fixture does little to conserve energy. Moreover, to work, the construction is dependent upon “light leaving the aperture collected as a fine beam”, i.e. a laser.
All lasers, as opposed to other light sources, emit collimated, concentrated and coherent light. Even if a laser beam is subsequently diffused, the light remains coherent, and is therefore always potentially harmful. The International Electrotechnical Commission's 60825-1 standard Safety of laser products, classifies lasers with an out power of 1.5 W as a class 4 laser. According to the Swedish Radiation Safety Authority, this means that just looking at an illuminated spot may be dangerous. Lasers used for surgery and metal cutting belong to class 4. Fixtures combining lasers and lenses, are therefore not appropriate for lighting. Moreover, a laser entails a construction of a certain size, inherently and because of the cooling required due to poor energy conversion of a laser. Therefore, such constructions are unlikely to endure any significant speed of rotation, and particularly, would not endure it for any considerable time.